Optical coupling device

ABSTRACT

An optical coupling device for coupling a light beam into a waveguide and a method of manufacturing the device. The device includes a grating portion having a plurality of essentially straight and essentially parallel scattering elements, wherein two or more of the scattering elements have different lengths. The method includes providing a grating layer on a substrate and forming a plurality of essentially straight and essentially parallel scattering elements from the grating layer, wherein two or more of the scattering elements have different lengths.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 from EuropeanPatent Application No. 08105147.6, filed Aug. 27, 2008, the entirecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an integrated optical device, forexample a coupling device for coupling incident light into a waveguide.Specifically, the present invention relates to an optical couplingdevice and a method for manufacturing an optical coupling device.

2. Description of the Related Art

Light wave transmission can occur along optical fibres. Light can alsobe propagated through planar waveguide structures that can beimplemented as photonic wave guides in integrated circuits. In manyapplications it is necessary to transmit an optical signal through aplanar waveguide structure and to optically couple the light into or outof the integrated optical chip.

When testing optical circuitry on a chip it can be desirable to couplelight from a core of an optical fibre, either perpendicularly or at anangle greater than zero degrees, into the integrated waveguide structureof the chip. This means that an efficient mechanism for couplingincident light into fabricated waveguides on a semiconductor wafer isdesirable.

Not only is the coupling an issue but also the size mismatch of thelight beam coming from, for example, a single mode fibre and the mode inthe photonic waveguide implemented on a chip. The dimension of theintegrated planar waveguide typically is much smaller than a light spotproduced by an optical fibre.

In the past, cleaved facets on a semiconductor substrate surrounded bypolymer tapers have been used to funnel the large light spot produced bya single mode optical fibre into the waveguide. Usually, the planarwaveguide and the incident light beam are arranged in parallel. Suchcleaved facet polymer tapers require large dimensions for deliveringsufficient efficiency. However, the testing of such optical circuits ona wafer is difficult to achieve because of inconvenient lateral in-planecoupling.

Other conventional coupling mechanisms employ grating couplers withattached linear tapers. Hence, two separate devices are formed next toeach other. A grating coupler comprises an array of parallel gratingelements which are arranged on a substrate. Incident light, for examplestemming from an optical fibre, is radiated perpendicularly or at anangle greater than zero degrees onto the grating plane. Throughscattering, the light can than be coupled into the plane of the gratingand fed into a photonic waveguide on the substrate.

However, the grated array is much larger than the diameter of thewaveguide. Therefore, an adiabatic tapering employing an appropriatedevice is additionally used. Conventional gratings cover areas of around10 by 10 μm wherein an adiabatic taper requires an additional length ofabout 500 μm until the modal size produced by the grating matches themodal size of the integrated waveguide of the respective optical chip.It is generally desirable to minimize the area assumed by such couplingand tapering devices.

U.S. Pat. No. 7,260,293 ('293) discloses an optical waveguide gratingcoupler that has a varying scatter cross section. This structure hasbent gratings that correspond to curved wave fronts of light coming froman attached waveguide. In '293 two layer stacks are needed since thegratings are arranged on top of a funnel-shaped portion that merges intoa corresponding waveguide.

In Michael M. Spühler, et al., “A very short planar silica spot-sizeconverter using a nonperiodic segmented waveguide”, Journal of LightwaveTechnology, vol. 16, No. 9, September 1998, page 1680 (Spühler), asegmented waveguide structure with an irregular tapering is introducedfor laterally coupling light into a photonic wave guide. Spühler aims atintegrating a spot-size converter with a waveguide. As mentioned above,lateral coupling may not be appropriate for testing a plurality ofintegrated optical circuits on a wafer.

Thus, it is desirable to design an improved optical coupling device.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an improved optical coupling device.

According to an aspect of the present invention, there is disclosed anoptical coupling device for coupling a light beam into a waveguide. Thedevice including a grating portion having a plurality of essentiallystraight and essentially parallel scattering elements, wherein two ormore of the scattering elements have different lengths.

According to another aspect of the present invention, there is discloseda method of manufacturing an optical coupling device for coupling alight beam into a waveguide. The method including the steps of:providing a grating layer on a substrate; and forming a plurality ofessentially straight and essentially parallel scattering elements fromthe grating layer, wherein two or more of the scattering elements havingdifferent lengths.

An advantage is that standard lithography processes can be used. Thereis no need to cleave any samples which facilitates wafer testing. Theentire coupling device can be realized employing only one layer. Hence,only one lithography and etching step is required. However, additionaldeposition and etch steps for producing a varying thickness can beemployed. Additionally, the grating can be easily aligned.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows a top view of an embodiment of an optical coupling device;

FIG. 2 shows a perspective view of a second embodiment of an opticalcoupling device;

FIG. 3 shows several process steps for manufacturing an embodiment of anoptical coupling device;

FIG. 4 shows a perspective view of a third embodiment of an opticalcoupling device;

FIG. 5 shows a perspective view of a fourth embodiment of an opticalcoupling device;

FIG. 6 shows a perspective view of a fifth embodiment on an opticalcoupling device; and

FIG. 7 shows a perspective view of a device implementing embodiments ofthe optical coupling device.

DETAILED DESCRIPTION OF THE INVENTION

In the following, preferred embodiments of an optical coupling deviceand method of manufacturing the device according to the presentinvention are presented with reference to the figures. In the figures,like or functionally like elements have been assigned the same referencesymbols.

According to an aspect of the present invention, there is disclosed anoptical coupling device for coupling a light beam into a waveguide, thedevice including a grating portion having a plurality of essentiallystraight and essentially parallel scattering elements, wherein two ormore of the scattering elements have different lengths.

The device can be regarded as an integrated grating coupler andspot-size converter. A plurality the scattering elements have differentelongation and in a preferred embodiment all of the scattering elementshave different lengths. By means of the differing lengths of thescattering elements, light is scattered at the front ends of thescattering elements and funneled or tapered towards a potentiallyattached photonic waveguide. Different elongations of scatteringelements, grating members, grating elements or segments correspond to alateral tapering.

It is to be understood that coupling essentially occurs not in-plane,i.e., the coupling essential does not occur parallel to the plane of thescattering elements. Rather, light is coupled onto the optical device ata non-zero angle between the incident light and the plane of the device,wherein the grating portion can be regarded as being essentially planarand defining the plane of the device. Light enters the grating portionwith a non-vanishing angle with respect to the plane of the gratingportion. A respective incident light beam can be coupled normal to saidplane. For example, a light beam may be coupled at an angle of 45degrees with respect to a normal vector of the grating plane.

It is understood that coupling can be either from a fibre through theoptical coupling device into a chip including a photonic waveguide orcoupling can be from a photonic waveguide of, or in connection with, achip, to a sensor above the grating portion of the optical device. It isan advantage of the proposed optical coupling device that a considerableamount of chip area can be saved. Further, the device enables to performwafer testing because perpendicular coupling or coupling at an angle oflight into or from an optical device on a wafer is facilitated.

In one embodiment of the optical coupling device a waveguide portion isarranged next to the grating portion wherein the waveguide portion andthe grating portion are arranged in the same plane, and the light beamis radiated onto that plane. For example, the scattering element next tothe waveguide portion which waveguide portion may comprise a ridgewaveguide has the essentially same length as is the width of the planarwaveguide portion.

The grating portion and/or the waveguide portion can be arranged on asubstrate. In a particular embodiment, the grating portion comprisesonly one layer. For example, the grating portion may be formed by atleast partially etching a grating material deposited on a substrate.Hence, a proposed optical coupling device can be manufactured bystandard CMOS processes.

Optionally, a substrate may comprise dielectric layers forming adielectric Bragg reflector below the grating portion. Dielectric Braggreflectors, or distributed Bragg reflectors, improve the couplingefficiency of the device. In such instance a deposition of the gratingmaterial for forming the grating portion is still considered to be onthe substrate although the material is not in direct contact with thesubstrate but rather the at least one dielectric layer.

The scattering elements of the grating portions are preferablyimplemented such that the electromagnetic waves of the incident lightbeam are converted to a modal size corresponding to the waveguide. Inparticular, the lengths and the arrangement of the scattering elementsare preferably determined by employing evolutionary optimizationprocedures that simulate the scattering processes. Such proceduresprovide suggestions where the front ends of the scattering elements needto be arranged in order to effect focusing of the incident light beam onthe waveguide.

In another embodiment, a scattering element has a width different to thewidth of the neighbouring scattering element. In a further embodiment,each scattering element has a width different to the widths of itsneighbouring scattering elements. Therefore, the width of a scatteringelement does not need to be uniform across the grating portion but canbe adapted according to the needs for the scattering at the front ends,for example, according to an optimization procedure. In anotherembodiment, at least two distances between neighbouring scatteringelements can be different.

In another embodiment, the cross section dimension of the gratingportion is larger than the cross section dimension of the light beam.Efficient coupling can be assured when the entire light intensity of thelight beam is radiated on the grating portion.

In a preferred embodiment, a front end of a scattering element has anon-planar shape.

According to a preferred embodiment the forming of the scatteringelements includes providing a mask for a pre-structured deposition ofthe grating layer on the substrate; and removing of the mask therebyforming a plurality of essentially straight and parallel scatteringelements. Preferably, a cladding layer can be provided finally.

FIG. 1 shows a top view of an embodiment for an optical coupling device.The optical coupling device 1 includes a grating portion 2 and a waveguide portion 3. The coupling device 1 can be used in applications wherea high physical integration density is desirable, for example,chip-to-chip optical interconnects. Coupling device 1 enables couplingof incident light, for example, stemming from an optical fibre. Incidentlight 18 is radiated onto the plane of the grating portion 2 and isdiffracted at scattering elements 5-17 as described below.

The scattering elements 5-17 are provided essentially in parallel toeach other. In this disclosure it is understood that essentiallyparallel means parallel within a margin of tolerance. An ideal gratinghas parallel gratings but a deviation is acceptable without altering thediffractive function of the grating. For example, in certainapplications an angle between grating elements of up to 5 degrees maystill qualify as being “essentially parallel”. Similarly, the term“essentially straight” is to be understood as “having no significantturnings” or “not being bent”. However, inaccuracies with respect to thestraightness of the grating elements within a margin of tolerance andwithout inferring the actual optical function of the gratings areacceptable. A person skilled in the art understands that microscopicallythe gratings may show deviations from ideal parallelism andstraightness.

Light enters the plane of the grating and runs parallel to this plane.Scattering occurs in particular at the front ends FE of the scatteringelements 5-17 and the light is funneled or fed into the waveguide 4 withhigh conversion efficiency. Thus, the disclosed structure captureselectromagnetic waves through the grating portion 2 which concurrentlyconverts the electromagnetic fields into a modal size that matches tothe attached strap waveguide.

The proposed optical coupling device 1 has the advantage that itresolves the problem of area consuming separation of a grating couplerand an attached spot-size converter, such as an adiabatic tapering.Instead, both functions are included in one compact device without anyloss of functionality or efficiency. Further, light can be irradiated orcoupled out from the grating portion more or less perpendicularly to theplane of the grating.

The geometric size of the scattering elements 5-17 is chosen as toprovide for a scattering pattern that can lead to modal match between awaveguide 4 and the attached grating portion 2. The waveguide 4 can be aridge waveguide or any integrated photonic waveguide. Grating portion 2has a cross section dimension DG which is chosen larger than the crosssection dimension DB of a potential incident light beam 18. The gratingor scattering elements shown in FIG. 1 have rectangular shape. Distancesbetween rectangular straight members do not need to be uniform. Forinstance, the distance D1 between scattering element 10 and scatteringelement 11 is different from the distance D2 between scattering element15 and scattering element 16. Also, the width W of the scatteringelements does not need to be uniform. Neighbouring scattering elements8, 9 for example have different widths W1, W2. In contrast toconventional grating couplers, the lengths or elongations of thescattering elements differ between at least some and preferably themajority of or even all of the scattering elements 5-17. This is becauseparticularly at the front ends FE of the scattering elements 5-17 lightscatters as to be prepared in an appropriate mode for entering thewaveguide 4 in the waveguide portion 3.

The grating portion 2 can span over a total area of 10 by 15 μm. Agrating is preferably placed on a substrate of silicon dioxide (burriedoxide) while the grating is made of silicon (top silicon), thus forminga silicon-on-insulator structure. The optical index of the material ofthe grating is preferably chosen higher than the optical index of thesubstrate material for efficient guiding. For example, the substratematerial can be silicon (also on top of silicon dioxide formingsilicon-on-insulator structure), silicon nitride, gallium arsenide,aluminium gallium arsenide, indium phosphide, gallium nitride, indiumgallium nitride, indium gallium phosphide, tantalum pentoxide, siliconor other materials.

One may use as guiding material, i.e. a top layer of the grating, amaterial having a higher refractive index than the substrate material.The guiding material may comprise silicon, thus forming asilicon-on-insulator structure, silicon nitride, gallium arsenide,aluminium gallium arsenide, indium phosphide, gallium nitride, indiumgallium nitride, indium gallium phosphide, tantalum pentoxide, hafniumoxide, titan dioxide, lutetium oxide, gadolinium oxide, barium strontiumtitanate, barium titanate, strontium titanite, strontium tantalite,strontium bismuth tantalite or other materials.

FIG. 2 shows a perspective view for a similar embodiment of the opticalcoupling device on a substrate 19. The substrate 19 may include any ofthe materials mentioned above. One can see that the boundary of thegrating portion including scattering elements 5-17 and 22-28 have anirregular shape. Therefore, incident light scatters at the front ends ofthe grating elements and can be converted for coupling into thewaveguide portion 4. The grating structure is fully etched through suchthat grating elements are separated from each other by a void space.

FIGS. 3A-3D show possible process steps in a method for manufacturing anembodiment of an optical coupling device. First, as shown in FIG. 3A, asubstrate 19 is provided. It is also possible that below the substrate19 a wafer stack can be arranged (not shown). Substrate 19, for example,includes BOX (burried oxide).

Next, as shown in FIG. 3B grating material 20 is deposited on thesubstrate 19. The grating material, for example, can be silicon.

Then, as shown in FIG. 3C, the silicon or grating material can belithographically etched to create the scattering elements 5-8 and/or apart of the ridge waveguide 4. Etching is not necessarily performedthrough either the substrate 19 or the grating material 20. One may alsochoose to only partially etch the grating structure as shown, forexample, between scattering elements 7 and 8. The etching can be doneconventionally creating vertical or near vertical trenches, but alsoslanted grooves or any advanced type of grating teeth than can be made.Scattering elements can correspond to grating teeth that compriseseveral steps of different geometry and size. Although FIGS. 3A-3C showcross sectional views, the overall geometry of the scattering elementscorrespond to the top views as shown in FIGS. 1 and 2.

As shown in FIG. 3D, optionally, the substrate may be implemented withseveral dielectric layers 21 forming a dielectric Bragg reflector ordistributed Bragg reflector. Also, in-plane additional first orderreflectors can be placed surrounding the grating portion and/or thewaveguide portion. Further, the device can be covered by a cladding thatpreferably reduces the index contrast of the material stack.

According to an alternative method for manufacturing an optical couplingdevice for perpendicular coupling or coupling at an angle of a lightbeam having a first cross section dimension into a waveguide having asecond cross section dimension the following steps are proposed:depositing a masking material onto a top-layer of a layered structure,the masking material having a composition different than the top-layerbeing a guiding layer, growing epitaxially or amorphously the samematerial as the over-layer and performing an etching as elaboratedregarding the previous implementation of a method for forming aplurality of essentially straight and parallel scattering elements,wherein the scattering elements have different lengths.

A further alternative method may further include the steps of: providinga substrate and applying a mask (e.g. photoresist), which is structured(e.g. by lithography) and then deposited either epitaxially oramorphously as the topmost layer to be the guiding material. Afterwards,the mask can be removed and a cladding may be applied. A person skilledin the art would also contemplate to combine certain manufacturing stepsof the above described variant of a method for manufacturing an opticalcoupling device.

FIG. 4 shows another embodiment of the optical coupling device whereintrenches between the scattering elements 5-17 and 22-28 are not entirelyetched through the substrate 19.

FIG. 5 shows yet another embodiment of an optic coupling device whereinscattering elements 5-17 and 22-25 are arranged in parallel with respectto their longitudinal axis but have non-planar shapes at some of thefront ends. In particular, the such “irregular” shape is prominent forgratings 3, 6, 8, 9, 10, 11, 12, 16 and 23. For example, the front end33 of scattering element 9 shows two parallel fingers extending from amain member which has rectangular size. Also, scattering element 16A,16B has a non-planar front end. For example, one may regard thisscattering element as comprising two scattering elements 16A and 16Bmerged or attached to each other along their elongation. One can seethat the scattering elements do not need to be uniform in length, width,shape or period. However, the scattering elements are essentiallyparallel to each other.

FIG. 6 shows yet another embodiment of an optical coupling device in aperspective view. FIG. 6 illustrates that scattering elements 5-17 and22-29 can have different width and distances with respect to each other.The actual shape of the scattering elements or members is determined bythe desired scattering pattern for incident perpendicular light or lightincident at an angle or light coming from the wave guide 4 into thegrating portion.

FIG. 7 shows a device 100 including an optical coupling device as shownabove. On a substrate 19, two combined spot-size converter and gratingcoupler devices 2, 30, or grating portions respectively, are arranged.The grating portions 2 and 30 are coupled through a waveguide 4 placedbetween them. Due to a coupling efficiency of about −3 dB, light can becoupled into the waveguide 4 by the first grating portion 2 and coupledout from the waveguide 4 by a second grating portion 30. FIG. 7 alsodepicts a single mode optical fibre 31 radiating light more or lessperpendicularly onto the first grating portion 2. The shown arrangementrefers to wafer testing where, for example, optical coupling devices orgrating portions are arranged on a semiconductor wafer that alsoincludes optical devices that are coupled to each other throughwaveguides. For testing the operation of optical integrated deviceslight can be coupled into the waveguide and transmitted to the relevantdevices or optical processing apparatuses and decoupled at a secondgrating portion 30.

All of the described implementations of optical coupling devices caneasily be obtained through standard CMOS fabrication processes. Insteadof several hundreds of microns taper length the proposed couplingdevices only cover areas of about 10 by 10 microns. The integratedspot-size converter and grating couplers provide for modal matching. Inone embodiment only one layer and thereby one lithography and etchingstep are required to obtain the grating elements on a substrate.Although the invention is disclosed with respect to certain preferredembodiment, variations having combinations of features can becontemplated without departing from the spirit and scope of theinvention.

1. An optical coupling device for coupling a light beam into awaveguide, the device comprising: a grating portion having a pluralityof essentially straight and essentially parallel scattering elements,wherein two or more of the scattering elements have different lengths.2. The device of claim 1, wherein the scattering elements form anintegrated grating coupler and spot-size converter.
 3. The device ofclaim 1, further comprising: a waveguide portion next to the gratingportion, wherein the waveguide portion and the grating portion arearranged in one plane.
 4. The device of claim 3, wherein the gratingportion and the waveguide portion are arranged on a substrate.
 5. Thedevice of claim 1, wherein the grating portion is formed by at leastpartially etching a grating material deposited on a substrate.
 6. Thedevice of claim 4, wherein the substrate comprises at least onedielectric layer.
 7. The device of claim 3, wherein the scatteringelements of the grating portion are configured to captureelectromagnetic waves of the light beam and convert the electromagneticwaves to a modal size corresponding to the wave guide.
 8. The device ofclaim 1, wherein one of the scattering elements has a width and at leastone neighbouring scattering element has a different width.
 9. The deviceof claim 1, wherein at least one distance between neighbouringscattering elements is different from at least one other distancebetween neighbouring scattering elements.
 10. The device of claim 1,wherein a cross section dimension of the grating portion is larger thana cross section dimension of the light beam.
 11. The device of claim 1,wherein at least one front end of at least one scattering element isnon-planar.
 12. A method of manufacturing an optical coupling device forcoupling a light beam into a waveguide, the method comprising the stepsof: providing a grating layer on a substrate; and forming a plurality ofessentially straight and essentially parallel scattering elements fromthe grating layer, wherein two or more of the scattering elements havedifferent lengths.
 13. The method of claim 12, wherein the step offorming of the scattering elements is carried out by: providing a maskfor a pre-structured deposition of the grating layer on the substrate;and removing of the mask to form the plurality of essentially straightand parallel scattering elements.
 14. The method of claim 13, furthercomprising: providing a cladding layer.